Clean map layers automatically

[Joonalai]

This PR:

• Cleans map layers automatically using pytest_runtest_teardown hook.
• Adds deprecation warning to clean_qgis_layer decorator.
• Fixes some broken tests with missing crs

Fixes #25.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (f6f2c02) 55.81% compared to head (c5fb22f) 56.46%.

Files	Patch %	Lines
src/pytest_qgis/utils.py	62.50%	9 Missing :warning:
src/pytest_qgis/pytest_qgis.py	60.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #45 +/- ## ========================================== + Coverage 55.81% 56.46% +0.64% ========================================== Files 7 7 Lines 464 487 +23 ========================================== + Hits 259 275 +16 - Misses 205 212 +7 ``` | [Flag](https://app.codecov.io/gh/GispoCoding/pytest-qgis/pull/45/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GispoCoding) | Coverage Δ | | |---|---|---| | [pytest](https://app.codecov.io/gh/GispoCoding/pytest-qgis/pull/45/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GispoCoding) | `56.46% <62.06%> (+0.64%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GispoCoding#carryforward-flags-in-the-pull-request-comment) to find out more.

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[Joonalai]

@LKajan, could you review this one?